A mechanical switching unit is typically triggered by way of an actuating element, such that the supply phase (the phase routed by way of the device) is hereupon interrupted by the mechanical switching unit. In order to trigger the mechanical switching unit and therefore to interrupt the electrically conductive connection between the output terminal and input terminal of the device (monitored supply phase), a mechanical force is exerted on the actuating element of the mechanical switching unit. As a result of the actuation of the actuating element of the mechanical switching unit a supply phase routed by way of the device is finally interrupted.
In thermal overload relays a thermomechanical tripping device (bimetallic tripping device) is used as a monitoring device and triggering unit in most cases on account of the favourable cost level. In order to monitor the motor or circuit, the bimetallic tripping device is placed by way of the thermal overload relay in the supply phase that is to be monitored. Since the bimetallic tripping device is situated in the supply phase (in the main current path of the load), it is heated to varying degrees as a function of the current flow present. If a thermal overload is present at the load, the increased current flow in the supply phase causes the bimetallic tripping device, in particular the bimetal thereof, to be deformed in such a way that a mechanical force is exerted on the actuating element of the mechanical switching unit by the bimetallic tripping device, as a result of which said actuating element is triggered. The monitored supply phase is consequently interrupted by means of the mechanical switching unit.
Also known are overload relays having electromagnetic triggering units in which the mechanical switching unit can be triggered by way of an electromagnetic tripping device of the triggering unit. It is possible to make a distinction in this context between two types of triggering units. There are triggering units which receive the triggering energy for actuating the actuating element of the mechanical switching unit directly from the triggering electronics of the triggering unit, and triggering units which are constructed as electromechanically triggered energy accumulators (maglatch). The latter have the advantage that the triggering electronics must provide less triggering energy than is actually needed for actuating the actuating element of the mechanical switching unit.
Independently thereof, the electromagnetic triggering units typically include a coil wound on a coil former, wherein the coil terminals must additionally be connected by way of lines (coil connecting lines) and/or plug-in connections to the electronics of the triggering unit.